Ball screw apparatus

ABSTRACT

In a ball screw apparatus, a ball train including a plurality of main balls is housed in a raceway between a ball track of a ball nut and a ball track of a ball screw shaft. A coil spring housed in the raceway includes a first end that engages with an end of the ball train and a second end supported by a stopper (a first recessed portion, a protruding portion, or the like) of the ball nut. A stopper ball having a diameter larger than the diameter of the main ball is interposed between the stopper and the second end of the coil spring.

INCORPORATION BY REFERENCE

This is a Division of U.S. application Ser. No. 14/807,041 filed Jul.23, 2015, which claims the benefit of Japanese Application No.2014-158165 filed Aug. 1, 2014 and Japanese Application No. 2014-242964filed Dec. 1, 2014. The disclosures of the prior applications are herebyincorporated by reference therein in their entireties.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a ball screw apparatus.

2. Description of Related Art

There has been proposed a ball screw apparatus with a ball traininterposed between a ball track in an outer periphery of a ball screwshaft and a ball track in an inner periphery of a ball nut. In the ballscrew apparatus, a coil spring is interposed between a ball at an end ofthe ball train and a stopper pin fixed to the ball nut (see, forexample, Published Japanese Translation of PCT Application No.2010-505072 (JP 2010-505072 A)). In such a non-circulating ball screwapparatus, the coil spring does not contract when the ball screw shaftis rotationally driven with a low axial load imposed on the ball screwshaft. Thus, the balls in the ball train do not move relative to theball nut. Therefore, the ball screw moves with the balls in the balltrain sliding on the ball screw shaft.

On the other hand, when the ball screw is rotationally driven with ahigh axial load imposed on the ball screw shaft, the coil springcontracts to allow the ball screw to move with the balls rolling withrespect to both the ball nut and the ball screw shaft. Thus, efficientscrew power transmission is achieved.

The coil spring is housed in a raceway between the ball screw shaft andthe ball nut. Thus, when the ball screw shaft is rotationally driven,the shape of the coil spring is likely to be distorted. Consequently,the coil spring may interfere with the ball crew shaft to cause wear orthe like.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a ball screw apparatusthat allows the orientation of the coil spring to be stabilized.

According to an aspect of the present invention, a ball screw apparatusincludes: a ball nut including an inner periphery, a ball track formedin the inner periphery, and a stopper; a ball screw shaft including anouter periphery and a ball track formed in the outer periphery, the ballscrew shaft being inserted through the ball nut; a ball train includinga plurality of main balls housed in a raceway formed between the balltrack of the ball nut and the ball track of the ball screw shaft; a coilspring including a first end that engages with at least one end of theball train and a second end supported by the stopper and housed in theraceway; and at least one stopper ball interposed between the stopperand the second end of the coil spring and having a larger diameter thanthe main ball.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features and advantages of the invention willbecome apparent from the following description of example embodimentswith reference to the accompanying drawings, wherein like numerals areused to represent like elements and wherein:

FIG. 1 is a schematic sectional view of a brake apparatus to which aball screw apparatus of a first embodiment of the present invention isapplied, illustrating a non-braking state;

FIG. 2 is an exploded perspective view depicting the configuration ofthe ball screw apparatus;

FIG. 3 is an enlarged sectional view of an enlarged part of FIG. 1,depicting the main part of the brake apparatus;

FIG. 4A is a sectional view of the ball screw apparatus in whichillustration of a ball screw shaft is omitted, and FIG. 4B is asectional view taken along line IVB-IVB in FIG. 4A and depicting a crosssection of the ball screw shaft;

FIG. 5 is a sectional view of the main part of the ball screw apparatus;

FIG. 6 is a schematic diagram illustrating a relation between a secondend of a first coil spring and a first stopper ball;

FIG. 7 is a schematic diagram illustrating a relation between a secondend of a second coil spring and a second stopper ball;

FIG. 8 is a sectional view of a ball screw apparatus of a secondembodiment of the present invention in which illustration of a ballscrew shaft is omitted;

FIG. 9A is a schematic sectional view of a second end of a ball nutincluded in a ball screw apparatus of a third embodiment of the presentinvention, and FIG. 9B is a schematic diagram illustrating a forceapplied to a stopper ball;

FIG. 10 is a schematic side view of an end surface of a second end of aball nut included in a ball screw apparatus of a fourth embodiment ofthe present invention;

FIG. 11 is a perspective view of the main part of a ball screw apparatusof a fifth embodiment of the present invention as viewed from an end ofthe ball screw apparatus closer to a disc;

FIG. 12 is a sectional view of the main part of the ball screw apparatusof the fifth embodiment of the present invention and depicting sectionalviews of a ball screw shaft, a ball nut, and a piston;

FIG. 13 is a sectional view of the main part of the ball screw apparatusof a sixth embodiment of the present invention and depicting sectionalviews of a ball screw shaft, the ball nut, and a piston; and

FIG. 14 is a schematic diagram illustrating a force acting on a stopperball included in a ball screw apparatus of an eleventh embodiment of thepresent invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below in detailwith reference to the attached drawings. FIG. 1 is a schematic sectionalview of a non-braking state of a brake apparatus 1 to which a ball screwapparatus 22 of a first embodiment of the present invention is applied.The brake apparatus 1 is an apparatus that applies a frictional brakingforce to a disc 2 rotating integrally with a wheel of an automobile orthe like

The brake apparatus 1 includes a floating caliper 3, a first backupplate 4 and a second backup plate 5, and a first pad 6 and a second pad7. The caliper 3 is movably supported by, for example, a knuckle (notdepicted in the drawings). The first backup plate 4 and the secondbackup plate 5 are disposed so as to sandwich the disc 2 between thefirst backup plate 4 and the second backup plate 5, and supported by thecaliper 3 so as to be able to move closer to and move away from eachother. The first pad 6 and the second pad 7 are fixed to the firstbackup plate 4 and the second backup plate 5, respectively, and canpress respective side surfaces of the disc 2.

The caliper 3 includes a first body 8, a second body 9, and a cover 10.The first body 8 and the second body 9 are fixed together. The cover 10is fixed to the second body 9. The first body 8 includes a body portion11 and an arm portion 12. One end of the second body 9 is fixed to thebody portion 11. The arm portion 12 is coupled orthogonally to the bodyportion 11. The second backup plate 5 is fixed to the arm portion 12.The second body 9 includes a cylinder 13 (corresponding to a brakecylinder) and an extension plate 14. The cylinder 13 is fixed to thebody portion 11 of the first body 8. The extension plate 14 extends fromthe cylinder 13.

The cylinder 13 has a first end 151 and a second end 152 that areopposite to each other in an axial direction AX. The cylinder 13includes a cylindrical portion 15 that is open at the first end 151 andan end surface plate 16 coupled to the second end 152 of the cylindricalportion 15. A piston (housing) 17 (corresponding to a brake piston) thatis movable in the axial direction AX is housed in the cylinder 13. Anend 171 of the piston 17 protrudes toward the disc 2 through an openingportion at an end of the cylinder 13 (that corresponds to the first end151 of the cylindrical portion 15) and is fixed to the first backupplate 4.

A seal member 18 is interposed between an outer periphery 17 a of thepiston 17 and an inner periphery of the cylinder 13 (that corresponds toan inner periphery 15 b of the cylindrical portion 15) to seal the gapbetween the outer periphery 17 a and the inner periphery. The sealmember 18 may be an O ring housed in a housing groove formed in theinner periphery of the cylinder 13 (the inner periphery 15 b of thecylindrical portion 15). The outer periphery 17 a of the piston 17 andthe inner periphery of the cylinder 13 (the inner periphery 15 b of thecylindrical portion 15) are coupled together via a key 19 provided inkeyways formed in the outer periphery 17 a and the inner periphery. Keycoupling using the key 19 allows movement of the piston 17 in the axialdirection AX to be guided and also allows rotation of the piston 17 withrespect to the cylinder 13 to be regulated.

A hydraulic pressure that biases the piston 17 toward the disc 2 may besupplied into the cylinder 13 through a hydraulic path not depicted inthe drawings. In that case, the cylinder 13 and the piston 17 form ahydraulic actuator. The caliper 3 functions to press both of the pads 6and 7 against the disc 2 to generate a braking force. The caliper 3includes an electric motor 20, a speed reduction apparatus 21, and theball screw apparatus 22. The speed reduction apparatus 21 reduces therotation speed of the electric motor 20. The ball screw apparatus 22converts rotary motion transmitted from the electric motor 20 via thespeed reduction apparatus 21 into linear motion of the piston 17 in theaxial direction AX.

The electric motor 20 includes a motor housing 23 and an output shaft24. The motor housing 23 is fixed to the extension plate 14 of thesecond body 9. The speed reduction apparatus 21 includes a driving gear25, an idle gear 26, and a driven gear 27. The driving gear 25 isattached to one end of the output shaft 24 of the electric motor 20 soas to rotate together with the output shaft 24. The idle gear 26 mesheswith the driving gear 25. The driven gear 27 meshes with the idle gear26. The idle gear 26 is pivotally supported by the second body 9 so asto be rotatable. The cover 10 is fixed to the second body 9 so as tocover the speed reduction apparatus 21.

FIG. 2 is an exploded perspective view depicting the configuration ofthe ball screw apparatus 22. In FIG. 2, a configuration including thepiston 17 is represented as the ball screw apparatus 22. As depicted inFIG. 1 and FIG. 2, the ball screw apparatus 22 includes a ball screwshaft 28, a ball nut 30, a retaining ring 40 (annular member), and thepiston 17. The ball screw shaft 28 is an input member. The ball nut 30is a rotatable output member screwed on the ball screw shaft 28 via aplurality of main balls 29. The ball screw shaft 28 is inserted throughthe ball nut 30. The ball screw shaft 28 is supported by the second body9 so as to be immovable in the axial direction but to be rotatable. Theball nut 30 is supported by the second body 9 so as to be movable in theaxial direction and to be non-rotatable.

As depicted in FIG. 1, specifically, the ball screw shaft 28 issupported by a rolling bearing 32 held in a support hole 31 formed inthe end surface plate 16 of the cylinder 13 such that the ball screwshaft 28 is rotatable and immovable in the axial direction (axialdirection AX). The driven gear 27 is coupled to an end 281 of the ballscrew shaft 28 so as to rotate together with the ball screw shaft 28.FIG. 3 is a sectional view of an enlarged part of FIG. 1. As depicted inFIG. 3, the ball nut 30 has an outer periphery 30 a and an innerperiphery 30 b. A ball track 33 is formed in the inner periphery 30 b.The ball screw shaft 28 has an outer periphery 28 a in which a balltrack 34 is formed. The main balls 29 forming a train are interposedbetween the ball track 33 and the ball track 34.

The outer periphery 30 a of the ball nut 30 includes a rotationregulation portion 36 and a cylindrical surface portion 38. The rotationregulation portion 36 is engaged with a rotation regulation portion 35of an inner periphery 17 b of the piston 17. The cylindrical surfaceportion 38 is fitted on a cylindrical surface portion 37 of the innerperiphery 17 b of the piston 17. The engagement between the rotationregulation portions 35 and 36 regulates rotation of the piston 17 andthe ball nut 30 relative to each other.

The ball nut 30 includes a first end 301 closer to the disc 2 and asecond end 302 that is on the opposite side from the first end 301, inthe axial direction AX (ball nut axial direction).

The first end 301 of the ball nut 30 is in contact with a positioningstep portion 39 of the inner periphery 17 b of the piston 17. Theretaining ring (annular member) 40 fitted in an annular groove formed inthe inner periphery 17 b of the piston 17 is engaged with an end surface303 of the second end 302 of the ball nut 30. Thus, the piston 17 andthe ball nut 30 are coupled together so as to move together in the axialdirection AX.

When rotation of the output shaft 24 of the electric motor 20 istransmitted to the ball screw shaft 28 via the speed reduction apparatus21 to rotate the ball screw shaft 28, the ball nut 30 moves in the axialdirection AX. At this time, the piston 17 is guided by the key 19 andmoves together with the ball nut 30 in the axial direction AX. FIG. 4Ais an enlarged sectional view of the ball nut 30. FIG. 4B is a schematicsectional view taken along line IVB-IVB in FIG. 4A.

As depicted in FIG. 4A, the main balls 29 held in the ball track 33 inthe ball nut 30 form a ball train L arranged along a raceway K formedbetween the ball track 33 and the ball track 34. The ball train Lincludes a first end La and a second end Lb. The ball screw apparatus 22includes a first coil spring 51 and a second coil spring 52 disposed onthe respective opposite sides of the ball train L in the raceway K.

The first coil spring 51 includes a first end 511 that engages with amain ball 29 a at the first end La of the ball train L and a second end512. The second coil spring 52 includes a first end 521 that engageswith a main ball 29 b at the second end Lb of the ball train L and asecond end 522. As depicted in FIG. 4B, a first recessed portion(recessed portion) 60 serving as a first stopper is formed in the innerperiphery 30 b of the ball nut 30. The first recessed portion 60 is openin the end surface 303 of the second end 302 of the ball nut 30. Thefirst recessed portion 60 is formed, for example, by forging in theaxial direction AX.

The ball screw apparatus 22 includes a first stopper ball 81 held in thefirst recessed portion 60 (first stopper) and interposed between thefirst recessed portion 60 and the second end 512 of the first coilspring 51. The second end 512 of the first coil spring 51 is supportedby the first recessed portion 60 via the first stopper ball 81. Asdepicted in FIG. 4A and FIG. 5, the ball screw apparatus 22 includes aprotruding portion 70 that protrudes into the raceway K and that servesas a second stopper. The protruding portion 70 is a part of a stopperpin 700 penetrating the ball nut 30 and is formed by the part of thestopper pin 700 that protrudes into the ball track 33.

The ball screw apparatus 22 includes a second stopper ball 82 housed inthe raceway K and interposed between the protruding portion 70 servingas a second stopper and the second end 522 of the second coil spring 52.The second end 522 of the second coil spring 52 is supported by theprotruding portion 70 (second stopper) via the second stopper ball 82housed in the raceway K. As depicted in FIG. 6, the diameter D1 of thefirst stopper ball 81 is larger than the inside diameter E1 of an endturn portion of the first coil spring 51 corresponding to the second end512 (D1>E1). The first stopper ball 81 functions to support the firstcoil spring 51 by being in contact with an inner periphery of the secondend 512 (end turn portion) of the first coil spring 51.

As seen in FIG. 4(A), the diameter D1 of the first stopper ball 81 islarger than the diameter of the main ball 29 (D1>d). The diameter D2 ofthe second stopper ball 82 may be the same as the diameter d of the mainball 29 (D2=d) or may be smaller than the diameter d of the main ball 29(D2<d).

As depicted in FIG. 7, the diameter D2 of the second stopper ball 82 islarger than the inside diameter E2 of an end turn portion of the secondcoil spring 52 corresponding to the second end 522 (D2>E2). The secondstopper ball 82 functions to support the second coil spring 52 by beingin contact with an inner periphery of the second end 522 (end turnportion) of the second coil spring 52. As seen in FIG. 4(A), thediameter D2 of the second stopper ball 82 may be equivalent to thediameter d of the main ball 29 (D2=d) or may be smaller than thediameter of the main ball 29 (D2<d).

The first recessed portion 60 of the ball nut 30 has a first surface 61and a second surface 62. The first surface 61 is a bearing surface thatsupports the second end 512 of the first coil spring 51 via the firststopper ball 81. The second surface 62 regulates movement of the firststopper ball 81 in the axial direction AX. The first surface 61 may becurved (for example, shaped like a circular arc) in an axiallyperpendicular cross section depicted in FIG. 4B. Although not depictedin the drawings, the first surface 61 may be defined by two planesintersecting each other. As depicted in FIG. 4A, the second surface 62may be a surface orthogonal to the axial direction AX.

When the ball screw shaft 28 is rotationally driven with a low axialload imposed on the ball screw shaft 28, the coil springs 51 and 52 donot contract. Thus, the main balls 29 of the ball train L do not moverelative to the ball nut 30. Therefore, the ball nut 30 moves with themain balls 29 of the ball train L sliding on the ball track 33. On theother hand, when the ball screw shaft 28 is rotationally driven with ahigh axial load imposed on the ball screw shaft 28, the coil springs 51and 52 contract. Thus, the ball nut 30 moves with the main balls 29 ofthe ball train L rolling with respect to both the ball nut 30 and theball screw shaft 28. Consequently, efficient screw power transmission isachieved.

According to the first embodiment, at least one of the first stopper(first recessed portion 60) and the second stopper (protruding portion70) (in the present embodiment, both the first and second stoppers)supports a corresponding one of the second ends 512 and 522 of the coilsprings 51 and 52 via a corresponding one of the stopper balls 81 and 82as depicted in FIG. 4A. Therefore, the orientations of the second ends512 and 522 of the coil springs 51 and 52 are stabilized to allowdistortion of the shapes of the coil springs 51 and 52 to be suppressed.As a result, the orientations of the coil springs 51 and 52 can bestabilized. This in turn allows suppression of wear resulting fromcontact between the ball screw shaft 28 and the coil springs 51 and 52.

Specifically, the first recessed portion 60 formed in the innerperiphery 30 b of the ball nut 30 and serving as the first stoppersupports the second end 512 of the first coil spring 51 via the firststopper ball 81 housed in the first recessed portion 60. Therefore, theorientation of the second end 512 of the first coil spring 51 isstabilized to allow the orientation of the first coil spring 51 to bestabilized.

The diameter D1 of the first stopper ball 81 is larger than the diameterd of the main ball 29 (D1>d). Thus, the first stopper ball 81 issuppressed from falling off from the ball nut 30 toward the ball screwshaft 28. Therefore, the first stopper ball 81 is stably held by thefirst recessed portion 60. Consequently, the orientation of the secondend 512 of the first coil spring 51 can be more stabilized.

As depicted in FIG. 6, the first stopper ball 81 functions to supportthe first coil spring 51 by being in contact with the inner periphery ofthe end turn portion of the first coil spring 51 corresponding to thesecond end 512. Thus, the orientation of the second end 512 of the firstcoil spring 51 can be more stabilized. As depicted in FIG. 4A, theprotruding portion 70 protruding into the ball track 33 and serving asthe second stopper supports the second end 522 of the second coil spring52 via the second stopper ball 82 housed in the raceway K. Therefore,the orientation of the second end 522 of the second coil spring 52 isstabilized to allow the orientation of the second coil spring 52 to bestabilized.

When the diameter D2 of the second stopper ball 82 is equivalent to thediameter d of the main ball 29 (D2=d), the second stopper ball 82 andthe main ball 29 may be the same ball. Therefore, the number ofcomponents is reduced to enable a reduction in manufacturing costs. Whenthe diameter D2 of the second stopper ball 82 is smaller than thediameter d of the main ball 29 (D2<d), the second stopper ball 82 issubjected to no load at the time of power transmission. Thus, theorientation of the second end 522 of the second coil spring 52 can bemore stabilized.

As depicted in FIG. 7, the second stopper ball 82 functions to supportthe second coil spring 52 by being in contact with the inner peripheryof the end turn portion of the second coil spring 52 corresponding tothe second end 522. Thus, the orientation of the second end 522 of thesecond coil spring 52 can be more stabilized. Alternatively, althoughnot depicted in the drawings, two or more first stopper balls 81 may beinterposed between the first recessed portion 60 serving as the firststopper and the second end 512 of the first coil spring 51 in the firstembodiment. Two or more second stopper balls 82 may be interposedbetween the protruding portion 70 serving as the second stopper and thesecond end 522 of the second coil spring 52 in the first embodiment.FIG. 8 is a sectional view of a ball screw apparatus 22A of a secondembodiment. Illustration of the ball screw shaft is omitted in FIG. 8.The ball screw apparatus 22A of the second embodiment in FIG. 8 isdifferent from the ball screw apparatus 22 of the first embodiment inFIG. 4A in that the second stopper ball 82 is removed and in that thesecond end 522 of the second coil spring 52 is directly engaged with andsupported by the protruding portion 70 (second stopper).

Components of the second embodiment in FIG. 8 which are the same asthose of the first embodiment in FIG. 4A are denoted by the samereference numerals as those in the first embodiment. For the firststopper ball 81, the second embodiment can also produce the same effectsas those of the first embodiment. Although not depicted in the drawings,two or more first stopper balls 81 may be interposed between the firstrecessed portion 60 serving as the first stopper and the second end 512of the first coil spring 51 in the second embodiment. FIG. 9A is aschematic sectional view of the second end 302 of the ball nut 30included in a ball screw apparatus 22B of a third embodiment of thepresent invention. FIG. 9A is taken along line equivalent to the lineIVB-IVB in FIG. 4A.

Components of the third embodiment in FIGS. 9A and 9B which are the sameas those of the first embodiment in FIG. 4A are denoted by the samereference numerals as those in the first embodiment. The ball screwapparatus 22B of the third embodiment in FIGS. 9A and 9B is differentfrom the ball screw apparatus 22 of the first embodiment in FIG. 4A inthat, as the first stopper, a first recessed portion (recessed portion)60B is formed instead of the first recessed portion 60. The firstrecessed portion 60B supports the second end 512 of the first coilspring 51 via the first stopper ball 81. The first recessed portion 60Bhas a substantially circular arc shape in an axially perpendicular crosssection depicted in FIG. 9A.

The first recessed portion 60B includes a first surface 61B, a secondsurface 63B, and a regulation surface (not depicted in the drawings).The first surface 61B is a bearing surface that supports the second end512 of the first coil spring 51 via the first stopper ball 81. Thesecond surface 63B intersects the first surface 61B to define a bottomsurface of the first recessed portion 60B. The regulation surfaceintersects each of the first surface 61B and the second surface 63B toregulate movement of the first stopper ball 81 in the axial direction AX(see FIG. 4A or the like). The second surface 63B is, for example,planar, and intersects the first surface 61B at an acute angle. Thesurface of the first stopper ball 81 is in point contact with the firstsurface 61B at a first contact point 611, and with the second surface63B at a second contact point 631. The first recessed portion 60B isopen in the end surface 303 (see FIG. 4A or the like) at the second end302 of the ball nut 30 and is formed by forging in the axial directionAX, as is the case with the first recessed portion 60.

The first stopper ball 81 is preferably held by the ball nut 30 so asnot to cause a backlash in a ball nut radial direction RD. As a resultof a backlash of the first stopper ball 81 in the ball nut radialdirection RD, the first stopper ball 81 may move in the ball nut radialdirection RD and may be brought into contact with the ball screw shaft28, which may cause wear. FIG. 9B is a schematic diagram illustrating aforce acting on the first stopper ball 81. As depicted in FIG. 9B, thefirst stopper ball 81 to which a spring force S1 from the first coilspring 51 (the force toward the center C of the first stopper ball 81)is applied is subjected to a reaction force C1 applied from the firstsurface 61B (the force toward the center C of the first stopper ball 81)and a reaction force C2 applied from the second surface 63B (the forcetoward the center C of the first stopper ball 81). If the reaction forceC2 applied from the second surface 63B is present (if the reaction forceC2 is larger than zero), the first stopper ball 81 is pressed againstthe second surface 63B.

In the third embodiment, a distance DT between the first contact point611 and the second contact point 631 in a normal direction ND is setlarger than the radius R of the first stopper ball 81 (=D1/2) (DT>D1/2).Thus, the reaction force C2 is present which is applied to the secondcontact point 631 of the first stopper ball 81 from the second surface63B. If the reaction force C2 applied to the second contact point 631from the second surface 63B is present (if the reaction force C2 islarger than zero), the first stopper ball 81 is pressed against thesecond surface 63B.

From another viewpoint, the spring force S1 from the first coil spring51 toward the center C of the first stopper ball 81 is applied to apredetermined position in an intermediate area M (depicted by a thickline in FIG. 9B) between a first opposite position 612 that is 180degrees opposite to the first contact point 611 across the center C ofthe first stopper ball 81 and a second opposite position 632 that is 180degrees opposite to the second contact point 631 across the center C ofthe first stopper ball 81 in a cross section perpendicular to the axialdirection (axial direction AX) of the ball nut 30. In this case, thefirst stopper ball 81 is pressed against the second surface 63B.

In addition to advantageous effects equivalent to those of the firstembodiment, the third embodiment produces the following advantageouseffects. That is, the spring force S1 from the first coil spring 51 isapplied to the intermediate area M between the first opposite position612 and the second opposite position 632 in the surface of the firststopper ball 81. Thus, the first stopper ball 81 can be kept pressedagainst the second contact point 631. This prevents the first stopperball 81 from moving toward the ball screw shaft 28. Consequently, thefirst stopper ball 81 can be held without a backlash in the ball nutradial direction RD.

Furthermore, the distance DT between the first contact point 611 and thesecond contact point 631 in the normal direction ND is set larger thanthe radius R of the first stopper ball 81. The reaction force C2 ispresent which is applied to the second contact point 631 of the firststopper ball 81 by the second surface 63B. Consequently, the firststopper ball 81 can be effectively pressed against the second surface63B. As a result, a backlash of the first stopper ball 81 in the ballnut radial direction RD can be more effectively suppressed.

In addition, the first stopper ball 81 can be prevented from movingtoward the ball screw shaft 28. As a result, the first stopper ball 81can be effectively suppressed from falling off from the ball nut 30.FIG. 10 is a schematic side view of the end surface 303 of the secondend 302 of the ball nut 30 included in a ball screw apparatus 22C of afourth embodiment of the present invention.

Components of the third embodiment in FIG. 10 which are the same asthose of the third embodiment in FIG. 9A are denoted by the samereference numerals as those in the third embodiment. The ball screwapparatus 22C of the fourth embodiment in FIG. 10 is different from theball screw apparatus 22B of the third embodiment in FIG. 9A in that thefirst stopper ball 81 and/or the first coil spring 51 are/is preventedfrom falling off from the first recessed portion 60 of the first coilspring 51 by use of the retaining ring 40 (see FIG. 3 or the like) thatprevents the ball nut 30 from moving in the axial direction with respectto the piston 17 (see FIG. 3 or the like).

As described above, the retaining ring 40 is fitted in the annulargroove formed in the inner periphery 17 b of the piston 17. When theball screw apparatus 22C is in the assembled state, the retaining ring40 engages with an outer peripheral portion of the end surface 303 ofthe second end 302. In other words, the retaining ring 40 faces the endsurface 303 of the second end 302 of the ball nut 30 so as to extendalong the outer periphery of the ball nut 30.

A distance W between the outer periphery 28 a of the ball screw shaft 28(a ridgeline of a portion adjacent to the ball track 34) and an innerperipheral edge 40 a of the retaining ring 40 is set shorter than thediameter D1 of the first stopper ball 81 (W<D1) when the retaining ring40 is in place. Thus, the first stopper ball 81 and/or the first coilspring 51 can be prevented from falling off from the ball nut 30.Furthermore, the retaining ring 40 intended to prevent the ball nut 30from moving in the axial direction (axial direction AX (see FIG. 3 orthe like)) with respect to the piston 17 is also used as an annularmember for prevention of fall-off. This eliminates the need for aseparate annular member for prevention of fall-off. Consequently, thenumber of components and costs can be reduced.

The annular member for prevention of fall-off may be provided separatelyfrom the retaining ring 40 that prevents the ball nut 30 from moving inthe axial direction (axial direction AX) with respect to the piston 17.For the first recessed portion 60B, the fourth embodiment can alsoproduce the same effects as those of the third embodiment. In the fourthembodiment, with the first recessed portion 60B of the third embodimenttaken as an example of the recessed portion, the configuration has beendescribed which prevents the first stopper ball 81 and/or the first coilspring 51 from falling off from the first recessed portion 60B. However,as a recessed portion for fall-off prevention, another recessed portion(for example, the first recessed portion 60 (see FIG. 4A or the like))may be used. FIG. 11 is a perspective view of the main part of a ballscrew apparatus 22D of a fifth embodiment as viewed from an end of theball screw apparatus 22D closer to the disc 2 (see FIG. 1 or the like).In FIG. 11, a configuration is depicted in which the ball screw shaft 28(see FIG. 1 or the like) and the piston 17 (see FIG. 1 or the like) arenot provided. FIG. 12 is a sectional view of the main part of the ballscrew apparatus 22D of the fifth embodiment of the present invention.

Components in FIG. 11 and FIG. 12 which are the same as those of thefirst embodiment in FIG. 4A are denoted by the same reference numeralsas those in the first embodiment. The ball screw apparatus 22D of thefifth embodiment in FIG. 11 and FIG. 12 is different from the ball screwapparatus 22 of the first embodiment in FIG. 4A in that, as the secondstopper that supports the second stopper ball 82, a second recessedportion (recessed portion) 71 is formed instead of the protrudingportion 70. The second recessed portion 71 supports the second end 522of the second coil spring 52 via the second stopper ball 82. Thediameter D2 of the second stopper ball 82 supported by the secondrecessed portion 71 is set larger than that of the main ball 29 (D2>d).

The second recessed portion 71 is open in an end surface 304 at thefirst end 301 of the ball nut 30. The second recessed portion 71 iselongated along the ball screw axial direction (axial direction AX) andhas a length corresponding to the width of portions (for example, twoportions) of the ball track 33. In other words, the second recessedportion 71 is formed to extend from the end surface 304 of the first end301 to penetrate the portions (for example, two portions) of the balltrack 33 in the ball screw axial direction (axial direction AX). Thewidth of the second recessed portion 71 in a ball nut circumferentialdirection Y is larger than the diameter of the second stopper ball 82.That is, the second stopper ball 82 is housed in the second recessedportion 71.

The second recessed portion 71 has a bearing surface 72 (see FIG. 11), aregulation surface (see FIG. 12) 73, and a third surface 74. The bearingsurface 72 supports the second end 522 of the second coil spring 52 viathe second stopper ball 82. The regulation surface 73 intersects thebearing surface 72 to regulate movement of the second stopper ball 82 inthe axial direction AX. The third surface 74 intersects each of thebearing surface 72 and the regulation surface 73 to define a bottomsurface of the second recessed portion 71. The third surface 74 may beformed of such a flat surface as depicted in FIG. 12. Alternatively, thethird surface 74 may be formed of a circular arc surface centered arounda direction along the ball screw axial direction (axial direction AX).

The third surface 74 presses the second stopper ball 82 against the balltrack 34 in the ball screw shaft 28. In other words, the second stopperball 82 is sandwiched between the third surface 74 and the ball track 34of the ball screw shaft 28 (the ridgeline defining the ball track 34).Thus, movement of the second stopper ball 82 with respect to the ballnut 30 is regulated (the second stopper ball 82 is fixed). Consequently,the second stopper ball 82 can be held in the ball nut 30 without abacklash.

The movement of the second stopper ball 82 with respect to the ball nut30 may be regulated by sandwiching the second stopper ball 82 betweenthe regulation surface 73 of the ball nut 30 and the ball track 34 ofthe ball screw shaft 28 having the ridgeline extending in a lead angledirection. FIG. 13 is a sectional view of the main part of a ball screwapparatus 22E of a sixth embodiment of the present invention.

Components in FIG. 13 which are the same as those of the fifthembodiment in FIGS. 11 and 12 are denoted by the same reference numeralsas those in the first embodiment. The ball screw apparatus 22E of thesixth embodiment in FIG. 13 is different from the ball screw apparatus22D of the fifth embodiment in FIG. 11 and FIG. 12 in that, as thesecond stopper, a second recessed portion (recessed portion) 71A isformed instead of the second recessed portion 71. The second recessedportion 71A supports the second end 522 (see FIG. 11 or the like) of thesecond coil spring 52 (see FIG. 11 or the like) via the second stopperball 82. The following description takes, as an example, a case wherethe diameter D2 of the second stopper ball 82 supported by the secondrecessed portion 71A is larger than the diameter of the main ball 29(D2>d). However, the diameter D2 of the second stopper ball 82 may bethe same as the diameter d of the main ball 29 (D2=d) or may be smallerthan the diameter d of the main ball 29 (D2<d).

The second recessed portion 71A includes a recessed step portion 75 thatis continuous with the end surface 304 of the first end 301 of the ballnut 30. The depth of the recessed step portion 75 in the ball nut radialdirection RD is equivalent to the diameter of the second stopper ball82. That is, the recessed step portion 75 can house one second stopperball 82. As depicted in FIG. 1, the piston 17 is shaped like a bottomedcylinder that is closed at the end 171 of the piston 17 (the end closerto the disc 2). As depicted in FIG. 13, a protrusion 172 that protrudesin a direction away from the disc 2 (see FIG. 1) is provided at an outerperipheral portion of the end 171 of the piston 17. The protrusion 172has an opposite surface 173 facing the recessed step portion 75 (to beexact, a regulation surface 73A) of the second recessed portion 71A. Theopposite surface 173 may be annular or may be provided only in an areafacing the recessed step portion 75.

As depicted in FIG. 13, the recessed step portion 75 has a bearingsurface (not depicted in the drawings), the regulation surface 73A, anda bottom surface 74A. The bearing surface supports the second end 522 ofthe second coil spring 52 via the second stopper ball 82. The regulationsurface 73A intersects the bearing surface to regulate movement of thesecond stopper ball 82 in the axial direction AX. The bottom surface 74Aintersects each of the bearing surface and the regulation surface 73A todefine the bottom surface of the recessed step portion 75. The bottomsurface 74A may be formed of such a flat surface as depicted in FIG. 13.The bottom surface 74A may be formed of a circular arc surface centeredaround a direction along the ball screw axial direction (axial directionAX).

The regulation surface 73A presses the second stopper ball 82 againstthe opposite surface 173. In other words, the second stopper ball 82 issandwiched between the recessed step portion 75 and the opposite surface173. Thus, movement of the second stopper ball 82 with respect to theball nut 30 is regulated (the second stopper ball 82 is fixed).Consequently, the second stopper ball 82 can be held in the ball nut 30without a backlash.

The present invention is not limited to the first to sixth embodiments.Although not depicted in the drawings, in a seventh embodiment that is avariation of the fifth embodiment in FIG. 11 and FIG. 12, a recessedportion having the same configuration as the recessed portion thatsupports the first stopper ball 81 may be used as the stopper thatsupports the second stopper ball 82, and the diameter of the secondstopper ball 82 supported by the recessed portion may be larger than thediameter of the main ball 29. In that case, one of the first coil spring51 and the first stopper ball 81 may be removed.

Although not depicted in the drawings, in an eighth embodiment that is avariation of the second embodiment in FIG. 8, the second coil spring 52is removed, and the main ball 29 b at the second end Lb of the balltrain L is directly engaged with and supported by the protruding portion70. Although not depicted in the drawings, in a ninth embodiment that isa variation of the first embodiment in FIGS. 4A and 4B and a tenthembodiment that is a variation of the second embodiment in FIG. 8, aprotruding portion having the same configuration as the protrudingportion 70 (second stopper) is used as the stopper that supports thefirst stopper ball 81, instead of the first recessed portion 60.

FIG. 14 is a schematic diagram illustrating a force acting on the firststopper ball 81 included in a ball screw apparatus of an eleventhembodiment that is a variation of the third embodiment in FIGS. 9A and9B. Components in FIG. 14 which are the same as those of the thirdembodiment in FIGS. 9A and 9B are denoted by the same reference numeralsas those in the third embodiment. In the eleventh embodiment, theintersecting angle between the first surface 61B and the second surface63B is an obtuse angle, and thus, the distance DT between the firstcontact point 611 and the second contact point 631 in the normaldirection ND is set smaller than the radius R of the first stopper ball81 (DT<R). Even in this case, if the spring force S1 from the first coilspring 51 (see FIG. 9A or the like) toward the center C is applied tothe intermediate area M between the first opposite position 612 and thesecond opposite position 632 in the surface of the first stopper ball81, the first stopper ball 81 is pressed against the second surface 63B.

For example, it is assumed that the spring force acts on the areabetween the first opposite position 612 and the second contact point 631(indicated by “x” in FIG. 14) as viewed in a cross section orthogonal tothe axial direction of the ball nut 30 (axial direction AX (see FIG. 1or the like)) (the spring force in this case is represented as a springforce S2 in FIG. 14). In this case, the first stopper ball 81 is notpressed against the second surface 63B. This may result in a backlash ofthe first stopper ball 81 in the ball nut radial direction RD. Moreover,the first stopper ball 81 may fall off from the ball nut 30.

In contrast, in the eleventh embodiment, the spring force S1 from thefirst coil spring 51 toward the center C is applied to the intermediatearea M between the first opposite position 612 and the second oppositeposition 632 in the surface of the first stopper ball 81. Thus, thefirst stopper ball 81 is pressed against the second surface 63B.Therefore, even in this case, the first stopper ball 81 can be heldwithout a backlash in the ball nut radial direction RD. In addition, thefirst stopper ball 81 can be effectively prevented from falling off fromthe ball nut 30.

In each of the above-described embodiments, a third coil spring (notdepicted in the drawings) may be interposed at one or more positions inan intermediate portion of the ball train L. Various modifications maybe made to the present invention within the scope of the invention.

What is claimed is:
 1. A ball screw apparatus comprising: a ball nutincluding an inner periphery, a ball track formed in the innerperiphery, and a stopper; a ball screw shaft including an outerperiphery and a ball track formed in the outer periphery, the ball screwshaft being inserted through the ball nut; a ball train including aplurality of main balls housed in a raceway formed between the balltrack of the ball nut and the ball track of the ball screw shaft; a coilspring including a first end that engages with at least one end of theball train and a second end supported by the stopper and housed n theraceway; and at least one stopper ball interposed between the stopperand the second end of the coil spring and having a larger diameter thanthe main ball; the coil spring includes: a first coil spring including afirst end that engages with a first end of the ball train, and a secondend that is directly or indirectly supported by the first stopper; andthe first stopper includes a recessed portion formed in the innerperiphery of the ball nut, and the stopper ball includes a first stopperball held in the recessed portion and interposed between the recessedportion and the second end of the first coil spring; the recessedportion includes a first surface serving as a bearing surface thatsupports the second end of the first coil spring via the first stopperball and a second surface that intersects the first surface to define abottom surface of the recessed portion, and in a surface of the firststopper ball, an elastic pressing force from the first coil spring isapplied to an intermediate area between a first opposite position thatis on the opposite side of the first stopper ball from a first contactpoint at which the first stopper ball contacts the first surface, and asecond opposite position that is on the opposite side of the firststopper ball from a second contact point at which the first stopper ballcontacts the second surface.
 2. The ball screw apparatus according toclaim 1, wherein the stopper includes a first stopper and a secondstopper, the coil spring includes: a second coil spring including afirst end that engages with a second end of the ball train, and a secondend that is directly or indirectly supported by the second stopper, andthe stopper ball is interposed between at least one of the first stopperand the second stopper and the second end of a corresponding one of thefirst and second coil springs.
 3. The ball screw apparatus according toclaim 1, wherein a diameter of the first stopper ball is larger than aninside diameter of an end turn portion of the first coil springcorresponding to the second end, and the first stopper ball functions tosupport the first coil spring by being in contact with an innerperiphery of the end turn portion of the first coil spring.
 4. The ballscrew apparatus according to claim 1, wherein a distance between thefirst contact point and the second contact point in a normal directionat the second contact point is set longer than a radius of the firststopper ball.
 5. The ball screw apparatus according to claim 1, whereinthe recessed portion is open in an end surface of the ball nut, the ballscrew apparatus includes an annular member disposed along an outerperiphery of the ball nut and engaging with the end surface of the ballnut, and a distance between the outer periphery of the ball screw shaftand an inner periphery of the annular member is set shorter than adiameter of the first stopper ball.
 6. The ball screw apparatusaccording to claim 3, wherein the recessed portion is open in an endsurface of the ball nut, the ball screw apparatus includes an annularmember disposed along an outer periphery of the ball nut and engagingwith the end surface of the ball nut, and a distance between the outerperiphery of the ball screw shaft and an inner periphery of the annularmember is set shorter than a diameter of the first stopper ball.
 7. Theball screw apparatus according to claim 5, further comprising a housingwith an inner periphery to which the outer periphery of the ball nut isfitted, wherein the annular member includes a retaining ring attached tothe inner periphery of the housing and engaging with the end surface ofthe ball nut to prevent the ball nut from moving in an axial directionwith respect to the housing.
 8. The ball screw apparatus according toclaim 6, further comprising a housing with an inner periphery to whichthe outer periphery of the ball nut is fitted, wherein the annularmember includes a retaining ring attached to the inner periphery of thehousing and engaging with the end surface of the ball nut to prevent theball nut from moving in an axial direction with respect to the housing.9. The ball screw apparatus according to claim 1, wherein the secondstopper includes a protruding portion that protrudes into the ball trackof the ball nut, and the stopper ball includes a second stopper ballhoused in the raceway and interposed between the protruding portion andthe second end of the second coil spring.
 10. The ball screw apparatusaccording to claim 2, wherein the second stopper includes a protrudingportion that protrudes into the ball track of the ball nut, and thestopper ball includes a second stopper ball housed in the raceway andinterposed between the protruding portion and the second end of thesecond coil spring.
 11. The ball screw apparatus according to claim 3,wherein the second stopper includes a protruding portion that protrudesinto the ball track of the ball nut, and the stopper ball includes asecond stopper ball housed in the raceway and interposed between theprotruding portion and the second end of the second coil spring.
 12. Theball screw apparatus according to claim 9, wherein a diameter of thesecond stopper ball is larger than an inside diameter of an end turnportion of the second coil spring corresponding to the second end, andthe second stopper ball functions to support the second coil spring bybeing in contact with an inner periphery of the end turn portion of thesecond coil spring.
 13. The ball screw apparatus according to claim 10,wherein a diameter of the second stopper ball is larger than an insidediameter of an end turn portion of the second coil spring correspondingto the second end, and the second stopper ball functions to support thesecond coil spring by being in contact with an inner periphery of theend turn portion of the second coil spring.
 14. The ball screw apparatusaccording to claim 1, wherein the second stopper includes a recessedportion formed in the inner periphery of the ball nut, the stopper ballincludes a second stopper ball held in the raceway and interposedbetween the recessed portion and the second end of the second coilspring, and the recessed portion holds the second stopper ball such thatthe second stopper ball is sandwiched between the recessed portion andthe ball track of the ball screw shaft to regulate movement of thesecond stopper ball with respect to the ball nut.
 15. The ball screwapparatus according to claim 2, wherein the second stopper includes arecessed portion formed in the inner periphery of the ball nut, thestopper ball includes a second stopper ball held in the raceway andinterposed between the recessed portion and the second end of the secondcoil spring, and the recessed portion holds the second stopper ball suchthat the second stopper ball is sandwiched between the recessed portionand the ball track of the ball screw shaft to regulate movement of thesecond stopper ball with respect to the ball nut.
 16. The ball screwapparatus according to claim 1, further comprising a housing with aninner periphery to which the outer periphery of the ball nut is fitted,wherein the second stopper includes a recessed portion formed in theinner periphery of the ball nut, the recessed portion includes arecessed step portion that is continuous with an end surface of the ballnut, the stopper ball includes a second stopper ball held in the racewayand interposed between the recessed portion and the second end of thesecond coil spring, the housing further includes an opposite surfacethat faces the recessed step portion, and the recessed step portionholds the second stopper ball such that the second stopper ball issandwiched between the recessed step portion and the opposite surface toregulate movement of the second stopper ball with respect to the ballnut.
 17. The ball screw apparatus according to claim 2, furthercomprising a housing with an inner periphery to which the outerperiphery of the ball nut is fitted, wherein the second stopper includesa recessed portion formed in the inner periphery of the ball nut, therecessed portion includes a recessed step portion that is continuouswith an end surface of the ball nut, the stopper ball includes a secondstopper ball held in the raceway and interposed between the recessedportion and the second end of the second coil spring, the housingfurther includes an opposite surface that faces the recessed stepportion, and the recessed step portion holds the second stopper ballsuch that the second stopper ball is sandwiched between the recessedstep portion and the opposite surface to regulate movement of the secondstopper ball with respect to the ball nut.